Exploration

The superlative life of Earth's largest fish

Dr Simon J Pierce is a marine conservation biologist and underwater photographer from New Zealand. He is a co-founder and Principal Scientist at the Marine Megafauna Foundation, where he leads the global whale shark research programme, and a regional Co-Chair for the IUCN Shark Specialist Group.

Words & photograph by Dr Simon J Pierce

 

After 14 years of studying whale sharks I’ve never actually written up the stats on their world-beating amazingness in a single article. It’s time to change that.

Whale sharks are the world’s biggest fish. In fact, they’re the largest cold-blooded animal. Some truly gigantic whale sharks have been documented in fisheries. One, caught off Taiwan, was 20 m length and 34 tons in weight. Another shark from the same area weighed 42 tons, though its length apparently wasn’t recorded. An 18.8 m whale shark was landed in India, too. The second largest fish is the tiddly little basking shark, which only grows to around 12 m. Bless their ickle fins. 

Based on the fossil record, whale sharks are likely to be the biggest fish to have ever lived. Megalodon, a huge, extinct whale-munching predatory shark famous for battling Jason Statham, is thought to have reached around 18 m. The largest-discovered remains of a plankton-feeding Jurassic bony fish, Leedsichthys problematicus, indicate a maximum length of around 16.5 m.

Despite their immense size, whale sharks were historically regarded as a particularly enigmatic fish, rare and wonderful to encounter. Whale sharks were only documented by science in 1828, and only 320 encounters had been recorded by the 1980s. Even Jacques Cousteau only saw two. 

These days, they’re still wonderful, but we know a lot more about their habits. They’re not that mysterious. The key is to understand that whale sharks are highly food-motivated. They’re basically oceanic Labradors.

Whale sharks like to eat. They’re filter-feeders, limited by a softball-sized throat, but the ocean is home to plenty of tasty little critters. Shrimp, tuna eggs, krill, copepods, crab larvae… all highly munchable. These small marine animals, collectively known as “zooplankton”, comprise a large proportion of whale shark diet. Common knowledge seems to be that whale sharks don’t eat anything else. In fact, they’ll happily target small fishes, because anchovies are delicious. Stop grimacing, you heathen.

Of course, eating tiny animals requires specialised mouthparts. Whale sharks have an efficient filtration system that enables them to feed on fish eggs less than one millimetre in size. A medium-sized whale shark can filter over 600,000 litres of seawater an hour. To put that in perspective, it’s like 600,000 bottles of milk, making 40 million cups of tea. Enough to last the average British household about a week.

Filtering all that water takes a lot of energy, but it’s worth it. At a site like Isla Mujeres in Mexico, where hundreds of sharks gather to feed on tuna eggs, that same medium-sized shark would eat about 142.5 kg of tuna eggs per day, consuming around 43,000 kilocalories. I did the math – that’s equivalent to about 8 kg of Cadbury Dairy Milk chocolate. 

A feast like that is worth swimming thousands of kilometres for, and indeed a significant proportion of the Atlantic whale shark population turns up at Isla Mujeres every summer. 

Whale sharks are incredible migrators. They have to be. Warm, blue, tropical waters are a biological desert – there’s almost no food on the surface. Individual sharks routinely travel over 10,000 km each year, but retain an impressive ability to return to their favourite haunts. 

As an example of their spatial memory, a whale shark sighted off Utila in Honduras was seen again the following evening, 100 km away at Gladden Spit in Belize. There, it was waiting for a snapper spawning event which only occurs for a few days around the full moon in certain months. That shark clearly knew exactly where it needed to be, and how to get there.

It’s likely that whale sharks are using the earth’s magnetic field for navigation. In the open ocean they seem to orientate towards with volcanic islands and atolls, which create pronounced magnetic anomalies that the sharks are probably using as waypoints.

Taking a step back for a moment, it’s easy to see that whale sharks play an important role in the ecosystem. Each shark poop transports nutrients and energy from productive areas, like plankton blooms, to the desolate open ocean. That provides a vital service to the deep sea. The sharks also move energy through the water column, because…

Whale sharks are the deepest-diving fish. Tagged sharks have been recorded diving to at least 1,928 m, well over a mile in depth. That may well be to do with the magnetic navigation mentioned above – deep dives give the sharks a better positional fix – but there’s likely to be some feeding activity at depth, too. The open ocean may be boring at the surface, but there’s plenty going on in the mesopelagic zone a few hundred metres down. Literally trillions of lanternfishes, probably the most abundant vertebrate on the planet, live in this twilight zone. They’d make an excellent snack for a hungry whale shark.

Whale sharks prefer surface water temperatures over 21ºC (as do I). It can be far cooler at depth, with whale sharks occasionally facing near-freezing 2ºC. That would be too cold for the sharks to function if they couldn’t retain their body heat. Fortunately, another superlative awaits.

Adult whale sharks have the thickest skin – at up to around 30 cm – of any animal. That provides great protection from predators, as well as insulation. What would eat a 20 m shark anyway? Well, not much actually. Only a single species, the killer whale, is known to kill adult whale sharks, and it’s unlikely to occur often. Small whale sharks are vulnerable, though. Baby whale sharks have been found in the stomachs of blue sharks and blue marlin, respectively.

That thick skin has the unfortunate side effect of making whale sharks extremely hard to ultrasound. But why would we want to do that?

Only one pregnant female whale shark has ever been confirmed. This 10.6 m “megamamma” shark, caught off Taiwan in 1995, contained 304 cute little babies. That’s almost twice as many pups as any other shark species. The babies are born free-swimming at 40-60 cm long, and likely get no further assistance from their mother. Other than that, we know little about whale shark reproduction. In fact, adult sharks remain extremely hard to find.

Whale sharks show extreme “sexual segregation”. Males and females, and juvenile and adult sharks too, use completely different habitats. Most of the known feeding areas, such as Mafia Island in Tanzania, are home to juvenile male sharks from 4-8 m length. There, like many coastal feeding areas, the sharks disappear once they reach adulthood at around 8-9 m. We presume they’re moving into the open ocean, and rarely approach the coast over the remainder of their lives. And, on that topic…

Whale sharks are extremely long-lived. They grow veeeeery slowly. Two whale sharks, Stumpy and Zorro, have been returning to Ningaloo Reef in Australia for over 20 years. A research paper describing the sighting history of the two sharks notes that “their growth has been negligible over the past two decades.” 

To be fair, that’s not entirely unexpected. Most large shark species are long-lived and slow-growing. Greenland sharks, living in the cool Arctic and deep waters, grow less than one centimetre per year and can be over 6 m at full size. A 5 m female was estimated to be 392 years old. Female white sharks only become adults at about 33 years and are estimated to live to at least 73. Even small spiny dogfish are thought to live for at least 80 years. Large whale sharks are estimated to live for about 130 years. Greenland sharks probably win, but… whale sharks are spottier. Speaking of spots…

We don’t know how many whale sharks there are in the world. Every whale shark is identifiable based on their unique spot pattern. Over 8,000 tourists and researchers have submitted photos to a global sighting database (www.whaleshark.org), identifying over 10,000 individual whale sharks. Most of those have been juvenile males, however. Even the largest feeding areas are home to less than 2,000 sharks – and some popular tourist destinations, such as Tanzania and Honduras, seem to have less than 200. 

The low numbers of whale sharks mean we have to protect them from human threats, like fishing and boat strikes, to ensure their survival. That’s why I’ve spent over a decade working to understand and protect them.

I’m sure you can understand now why I think whale sharks are such exceptional fish. These inoffensive giants are out there breaking records, terrorising plankton, enhancing ecosystems, all without making a fuss. Colossal they may be, but their very existence embiggens us all.

 

Column by Simon J Pierce. 

Issue Seven
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This column appears in ISSUE 7: Frozen breath of Oceanographic Magazine

Issue Seven
Supported by WEBSITE_sponsorlogos_blancpain
Supported by WEBSITE_sponsorlogos_blancpain

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